I must assume that you meant "6 1/2" and not "61 / 2."
If each cupcase requires 1/3 ounce of flour, then the total whole cupcakes that can be made using the entire 6 1/2 oz. bag is
6 1/2 oz 13 oz 1 cupcake
----------------------- = ---------- * ---------------- = 39 cupcakes (answer)
1/3 oz / cupcake 2 (1/3 oz)
